Before being able to judge how much effect the fish "may" have had, there is quite a bit information missing. Any change in the overall water parameters will depend on how many new fish as well as how large the tank. The definition of "smaller group" is not clear as some might think of 3, while others may think 20!
The effect of three new fish in 10 gallons will be much greater than in 55 gallons.
But all that aside, I would not think melting of plants would be nearly as obviously caused by a sudden increase in bio load as fish death or disease! Any sudden increase in ammonia seems to effect fish far more than plants.
Perhaps other changes might be to blame? Anything like a pretty heavy cleaning or rearrangement that would disrupt the plants, even though it did not seem like much change? Any changes in fert dosing or CO2 use? Any light or schedule changes?
I might try to look for other causes.